1. Field of the Invention
The present invention relates to a multilayer wiring substrate that is formed by pressing and firing a plurality of insulating layers, and to a module including the multilayer wiring substrate.
2. Description of the Related Art
There is known so far, as illustrated in FIG. 5, a high-frequency switch module 100 including a high-frequency switch 102 that is flip-chip mounted to one principal surface of a multilayer substrate 101 (See, for example, International Publication WO2010-143471, especially paragraphs [0030]-[0032] and FIG. 4). In the high-frequency switch module 100, the multilayer substrate 101 is a ceramic multilayer substrate in which a dielectric ceramic and a wiring electrode 103 are alternately stacked. A plurality of mount electrodes 104 connected to various terminals of the high-frequency switch 102 is formed on one principal surface (upper surface) of the multilayer substrate 101, and a plurality of outer electrodes 105 connected to electrodes of one or more external circuit substrates is formed on the other principal surface (lower surface) of the multilayer substrate 101. Furthermore, a plurality of via conductors 106 for connecting the mount electrodes 104 and the wiring electrodes 103 and for connecting the wiring electrodes 103 and the outer electrodes 105 are disposed inside the multilayer substrate 101.
In general, a ceramic multilayer substrate is manufactured as follows. First, slurry containing mixed powder of, e.g., alumina and glass, together with an organic binder and a solvent mixed additionally thereto, is formed into sheets, thus preparing a plurality of ceramic green sheets. In each of the ceramic green sheets, via holes are formed at predetermined positions by laser processing, for example, and a conductive paste containing, e.g., Ag or Cu, is filled into the via holes, thus forming via conductors for interlayer connection. Furthermore, various electrode patterns are formed by printing the conductive paste. The multilayer ceramic substrate is then obtained by stacking the ceramic green sheets, and by pressing and firing a stacked multilayer body under a predetermined pressure at a predetermined temperature.
When the multilayer substrate 101 of the high-frequency switch module 100 of the related art is formed in accordance with the above-described method, the following problem may arise in some cases. Portions of the one principal surface of the multilayer substrate 101, which are overlapped with the outer electrodes 105 when looked at in a plan view of the multilayer substrate 101, are raised corresponding to thicknesses of the outer electrodes 105. Accordingly, a level difference (step difference) in a stacking direction of the multilayer body may occur in the one principal surface between the portions overlapping with the outer electrodes 105 formed on the other principal surface and other portions not overlapping with the outer electrodes 105.
In such a situation, when the high-frequency switch 102 is flip-chip mounted to a region of the one principal surface where the portion including the above-mentioned level difference is present, there is a possibility of a mounting failure that, due to a level difference between the mount electrodes 104 formed in the above-mentioned region, a failure of solder wetting may occur between some of the mount electrodes 104 and a corresponding terminal of the high-frequency switch 102, or the high-frequency switch 102 may tilt and displace from a proper position.